Firefox Addon Virus on Ubuntu Laptop?

Is it possible to get viruses through addons on an Ubuntu computer? Because otherwise Ubuntu doesn't actually have any viruses.

No, rather less. Linux controls the repositories similar to the Google Store

The addon refers to Firefox. So in general there can be problems

Not if the add-on is integrated via the official https://addons.mozilla.org/...ozilla.org.
There, add-ons etc. Are checked for viruses etc. Prior to publication.

It is quite possible to penetrate Linux-based operating systems - i.e. Debian derivatives such as Ubuntu or even the entire MacOS systems. Firefox add-ons or other browsers, like the browser itself, are usually trapped in a "sandbox" on modern systems from which they can't break out.
Long talk in a nutshell: don't worry about that, but still think about exactly which add-on you are installing and why ^^
If there's a special occasion for your question, you can also describe it.